(1) Field of the Invention
The present invention relates to a permanent current switch having a coiled superconducting wire and a heater wire, which is prepared using, in the superconducting wire, a magnesium diboride superconducting (hereinafter abbreviated as MgB2) wire (hereinafter referred to as the MgB2 wire) that exhibits a superconducting property in an environment not in excess of a critical temperature.
(2) Description of Related Art
The MgB2 wire has a critical temperature of 39 K, which is about 20 K or more higher than that of an ordinary metal-based superconducting material. The wire also has a high critical magnetic field, and this is said to be about 50 T on a thin film. In addition, since the wire has a remarkably small anisotropic magnetic field and an excellent flexural property, it has a very few differences from the ordinary metal-based superconducting material in preparing and handling of a wire material.
A permanent current switch is one of applications utilizing a very high thermal stability of this MgB2 wire.
A permanent current switch using this MgB2 wire is described in JP-A-2003-37303.
The permanent current switch is generally formed using an NbTi wire in which a CuNi alloy is used as a stabilization material. However, NbTi has a critical temperature as low as about 9 K, and has a temperature margin of only about 5 K in liquid helium having a critical temperature of 4.2 K. Therefore, the NbTi wire may be easily transferred to a normal conducting state even when small disturbance energy is applied. For such a reason, a permanent current switch has been developed using the MgB2 wire having a temperature margin of 35 K in liquid helium.
However, there has been a problem that even if the permanent current switch is prepared simply using the MgB2 wire, the switch has a resistance of about 1×10−10 Ω, and a highly reliable permanent current switch having a less resistance value cannot be prepared.